This invention relates to integrated circuit lithography, in particular, to a method of adjusting a lithography system or tool to enhance image quality correction by using a reduced exposure dose during setup of the lithography system or tool.
Electronic beam systems have been employed for micro-fabrication of large scale integrated circuits on semiconductor substrates. Typically patterns are written on radiation sensitive materials usually composed of photoresist, deposited on targets such as substrates in the form of semiconductor wafers (in what is referred to a xe2x80x98direct writexe2x80x99 lithography process) or photolithographic masks (for use in mask lithography). The electron beam exposes the radiation sensitive material and a pattern is developed on the wafer or in the mask. A typical electron beam system includes an electron beam source, a deflection system for deflecting the electron beam in a predetermined pattern and magnetic projection lenses for focusing the electron beam at the target.
The aforementioned lithography mask is used in projecting the pattern onto substrates, i.e, semiconductor wafers. More specifically, the pattern on the mask is optically projected by, e.g., ultraviolet light onto the wafer. This process is more common in high volume production applications.
Also, masks formed of a plate of material which is capable of blocking or scattering an electron beam have been used for lithography. Such masks are sometimes referred to as blocking masks or scattering masks. In a stencil mask application, the plate has apertures therethough defining the desired pattern. Another application known as a SCALPEL(trademark) scattering mask incorporates a thin layer of patterned highly scattering material on a thin membrane of weakly scattering material. An electron beam is projected at the mask in question and through the apertures or unclad membrane to project the patten onto substrates, i.e, semiconductor wafers.
Both direct write and projection lithography systems require adjustments during setup to optimize image quality (e.g., focus, astigmatism, etc.) and thereby maximize pattern resolution. Obviously, there are tradeoffs between resolution, cost and production requirements and pattern resolution is maximized in view of these often competing requirements.
By way of example, when maximizing pattern resolution in a system employing a mask (scattering or blocking mask) it is common to set the lithography system up using a test pattern. This test pattern would have lines or elements which are smaller (or finer) than the lines or elements of a production pattern. Lines of the production pattern may be, for example, 200 nm, whereby lines of the test pattern would be 100 nm. In this way, the lithography system is adjusted to optimize image quality (e.g., focus, astigmatism, etc.) based on the 100 nm lines, thus providing a higher sensitivity to misadjustments than if the system was adjusted using the 200 nm lines. This approach has worked well for many years. However, as the industry continues to demand smaller and smaller microscopic patterns it becomes more and more difficult to obtain the desired test pattern resolution. This is primarily due to limitations in manufacturing masks, in this case test masks are required to have patterns that are even smaller (or finer) than the now smaller demanded production patterns.
The above-discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by the method of adjusting a lithography system or tool to enhance image quality correction of the present invention. In accordance with the present invention, the method enhances image quality correction by using a reduced beam exposure dose during setup of the lithography system or tool.
A typical projection electron beam lithography system comprises an exposure column unit and a control unit. The exposure column unit generates a shaped beam and directs this shaped beam through a series of deflectors to a mask which is positioned on a movable stage. The control unit provides control management for the components of the exposure column unit. In the prior art, pattern resolution in the system was maximized using a test pattern on a mask having geometries which are smaller than the geometries of a production pattern on a mask. In the present invention, the system maximizes pattern resolution using a mask having test pattern geometries that are at least the same size as the geometries of the pattern of a production mask. However, unlike in the prior art, the beam exposes the radiation sensitive material on the test wafers for a period of time shorter than the nominal exposure. Adjustments to the system during setup are made in the same manner as in the prior art, e.g., current changes to lenses and correction coils, deflection positioning and stage positioning.
The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.